Films with high clarity, high stiffness and high dart impact are highly demanded in packaging industry. Clarity delivers aesthetics properties, high stiffness and high dart afford down-gauge properties.
Ethylene alpha-olefin copolymer resins or film structures comprised these resins that can make films with improved aesthetics (optical) and abuse resistance (dart) properties create value to end-users. Heterogeneous ethylene alpha-olefin copolymers have both high crystalline (thick crystals) and copolymer (thin crystals) fractions.
Optical properties of a film can be defined in terms of surface gloss, haze and clarity. Haze can depend on internal haze (bulk scattering) and external haze (surface scattering). Both external haze and internal haze can be a function of the high crystalline fraction content and high crystalline fraction molecular weight of the resin used to make the film. High crystalline fraction can be made of thick crystals that scatter light and hence an increase in the high crystalline fraction content increases the haze of the film and hurts its optical properties. Decreasing the molecular weight of the high crystalline fraction can increase the thickness of the crystals in the high crystalline fraction. The thicker the high crystalline fraction crystals, the worse are the film optics. Hence a reduced content and increased molecular weight of the high crystalline fraction would be desirable for improved optics, although too high of a molecular weight can cause melt fracture problems due to high melt elasticity. Also, increasing the molecular weight of the high crystalline fraction, for improved optics, could hurt dart properties of the film. Higher molecular weight of the copolymer fraction is desirable for higher film dart. For a particular resin melt index (MI or I2), increasing the molecular weight of the high crystalline fraction for improved optics, has to be balanced by lowering the molecular weight of copolymer fraction so that the MI be kept constant. This decrease in copolymer molecular weight will hurt the dart properties. Therefore an optimum high crystalline fraction molecular weight is desirable for balanced dart and optical properties. Thick crystals from the high crystalline fraction provide strength to the film improving its tear properties. Decreasing the high crystalline fraction content for improving optics could therefore hurt film tear. In order to achieve a balance of tear and optical properties an optimum content of high crystalline fraction is desirable.
Surface haze of the film can depend on the molecular weight distribution of the ethylene alpha-olefin copolymer. Very broad molecular weight distribution usually increases melt elasticity of the resin leading to surface melt fracture while very narrow molecular weight distribution can cause processing issues at the die causing surface melt fracture. Since presence of surface melt fracture hurts film optical properties, an optimum molecular weight distribution is also required for improved optical properties.
Dart properties of films can depend on the molecular weight distribution and on the copolymer fraction content. The narrower the molecular weight distribution and more the copolymer fraction content, the higher is the film dart. Too narrow of molecular weight distribution could hurt optical properties and processability (film fabrication) and hence an optimum molecular weight distribution is required for a balance of processability, dart and optical properties. Also, increase in copolymer fraction content could be achieved at a cost of reduction in high crystalline fraction content and this could hurt film tear. Therefore in order to achieve a good balance of processability, dart, tear and optical properties, a particular combination of molecular weight distribution and high crystalline and copolymer fraction content is required.
Stiffness of films depends on resin density. The higher the resin density, the more the crystalline fraction, and the higher the film stiffness accordingly. However, as discussed previously, high crystalline faction will hurt film optical properties and dart impact, so, film stiffness and film optical/dart impact are mutually exclusive properties.
The present invention is a multilayer film made from a particular resin family with this particular combination of molecular weight distribution and high crystalline and copolymer fraction content and molecular weight. For an equivalent density and melt index, this combination of resin characteristics produces a film with improved optics, stiffness, dart, tear and processability.